As known, in an urban environment, a cellular radio communication network comprises micro-cells (typically, less than 2 kilometers wide) covering limited areas. The base station of a micro-cell is typically located few meters (e.g. 5 m) above the ground. This implies that the radio waves emitted by the antenna of the base station, in an urban environment, can encounter lots of obstacles along their propagation path, such obstacles being represented mainly by buildings.
Typically, as the base station is located at a height which is much lower that the average height of the buildings, the radio wave emitted by the base station can interact with the buildings along its propagation path towards a generic point within the micro-cell. In particular, the radio wave may reach a point within the micro-cell along a direct propagation path between the base station and the generic point (the so called direct visibility path) but also along other paths which originate from the interaction of the radio wave with the buildings. In particular, the radio wave emitted by the base station may reach a point within the micro-cell after having been reflected by the vertical walls of the buildings and/or having been diffracted by the edges of the buildings, both vertical edges and horizontal edges (i.e. the edges on the roof).
A known technique typically used to estimate the electric field strength produced by an antenna in a specific area is the so called “ray tracing” which is based on the electromagnetic theory of Geometric Optics or Ray Optics. According to this known technique, once the position of the antenna is fixed, to determine the electric field strength at a point P within the area, all the possible propagation paths of the rays originated from the antenna and reaching the point P are considered. The propagation paths can comprise the direct path between the antenna and the point P, and all the other paths originating from reflections and/or diffractions of the rays on the obstacles encountered during their propagation from the antenna to the point P. The propagation paths comprising indoor routes, i.e. routes inside the buildings, are excluded.
An alternative technique which may be used to estimate the electric field strength produced by an antenna in a specific area is the so called “ray launching” technique. Once the position of the antenna is fixed, this technique provides for determining rays propagating from the antenna along a set of angular directions spaced by a given angular step. Considering a given direction, the propagation path of each ray along this direction may comprise a direct path (when the ray does not encounter any obstacle along the considered direction) and paths originating from reflections and/or diffractions of the ray on the obstacles encountered during its propagation along the direction. The points where the electric field strength is computed are the points belonging to the propagation path of each single ray.